Many readers will be aware of how much I love Krell audio gear and their class-A designs. The Krell KSA-150 Class A Power Amplifier is a great example of one of these hot, power-hungry designs. In this article, I discuss the very basics of class-A amplifiers, with service details and images of the gorgeous KSA-150!
You may have already watched my restoration video, where I completely refurbish one of these beasts.
The video has generated a lot of interest and I receive many queries about this restoration. Powerful class-A amplifiers like the Krell KSA-150 need regular inspection to insure they don’t go up in smoke. This is largely because of the prodigious heat they generate, a by-product of the class-A amplifier topology.
What is ‘Class-A’?
Class-A configuration means that the output devices are biased or “on” and working in the linear part of their transfer curve 100% of the time, whether there is a signal present or not. This is achieved by feeding current into the base of the transistors, biasing them so they never switch off. The quiescent current is defined as the current drawn by the device when zero signal voltage is present.
It helps to think of duty-cycle, or the number of degrees of phase of the music signal through which the transistors conduct. In a class B amplifier, the transistors conduct through 180 degrees of signal, or half of the waveform. With class-A amplifiers, the transistors conduct through 360 degrees of signal, ie continuously. In a pure class-A amplifier, the maximum peak load current should be conducted by all transistors at all times. Because a single transistor conducts through 360 degrees of music signal, one output device can reproduce the entire signal. This gives the simplest amplifier configuration of one output device per channel – singled ended class-A.
In many modern so-called class-A amplifiers, the output devices are arranged in push-pull pairs and set to conduct around 1/2 the peak load current at all times. This is not really class-A but actually a high-bias class-AB configuration. Class-AB operation brings power savings, but let purity and more parts.
Advantages and Disadvantages of Class-A
All this might sound wasteful, but the benefit of class A operation is that the signal swings from positive, through zero to negative with no crossover distortion at the crossover point. The transistors are always conducting and operating in their linear zone. This is not energy-efficient, but offers reduced distortion. In fact my KSA-150 constantly draws around 300 watts and dissipates that as heat into my listening room. This has some advantages on cool nights, but resistive electrical heating is not a cost-effective way to heat a room…
Naturally, transistors drawing several amps between them at all times produce lots of heat and heat kills capacitors. So capacitors tend to fail in class-A amplifiers and the KSA-150 is no exception. Krell also modified the protection relay driver circuitry over the years to reduce heat-related failure of parts on the protection PCB. When I service a KSA-150, I assess the state of these parts and upgrade them where necessary.
The Krell KSA-150
This particular customer KSA-150 was in excellent condition, my customer had just purchased it and wisely decided that an inspection was in order. My customer was sure that the amp was a locally supplied unit but the back plate specified 110VAC meaning it was not. The safest option was to open the chassis and inspect the line voltage configuration. In fact, this amp was set for 240V, which is as close as we can get to the 250VAC we see here in Western Australia.
The big Krells do suffer failures and they seem to be fairly predictable. Common failure points occur on the soft start board. There is a design flaw, with the inrush limiting resistors being under-rated and mounted too close to the PC board. They are also mounted in a limited space right next to some electrolytic capacitors and a protection relay.
These resistors run very hot, scorching the board and cooking everything located nearby. I had to change many parts on that board to restore normal function. Fortunately, these parts are easy to get and standard values. The KSA-150 produces a lot of heat and on every other board there are capacitors that need to be replaced if they haven’t been already.
I recommend a comprehensive overhaul of any KSA-150 or KSA-250 every 10 years or so to avoid costly damage from dying parts. I’ve seen first-hand the damage that can result when parts inside a big Krell go bad and it’s not pretty!